Firefox display warning message "This connection is not secure. Logins entered here can be compromised." as soon as you start typing username or password on any website. it is like below image.
Firefox display this message to user when he/she is trying to login to a web page not having SSL certificate/connection. The Message convey that if you enter your username or password it could be stolen by hackers and attackers.
This warning message started with Firefox version 52, and can be disabled by updating a parameter in config.
Please follow the step to remove the warning:
1) Open a new tab and type the url as about:config.
2) Click the button "I accept the risk!"
3) You will see list of Preferences.
4) Search for "security.insecure_field_warning.contextual.enabled" using search box.
5) Make Preferences value to "false" by double clicking on the row.
6) Now just refresh the login web page.
Warning will be removed. Even if it still showing the warning restart the firefox and check again.
Click here to see firefix Support page.
Hope it will help someone out there!!!